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Study: dead zebras in electrified cages show how anthrax spreads in wild

One way to figure out how anthrax spreads in the wild is to build electrified cages around zebras that have died in a Namibian national park.

Anthrax is a nasty disease caused by the bacteria B. anthracis. It infects animals as they graze, and they in turn spread it as they are eaten either by predators or scavengers.

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Its mortality rate varies depending on how the bacterial spores are absorbed, from 20 percent for contact with skin to as high as 75 percent if inhaled.

B. anthracis can live in the soil for decades, and it has historically caused hundreds of thousands of human deaths. That number has dropped considerably over the last century thanks to comprehensive vaccination programmes (and the discovery of effective antiobiotics), but it still costs a lot of money each year to keep B. anthracis out of the food chain.

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In wild environments such as those found in national parks, a lot of time and effort goes towards preventing scavengers from eating animals that have died from anthrax. Most of those scavengers -- such as jackals, hyenas and vultures -- have evolved immunities to B. anthracis, so they don't get sick from eating infected meat -- but the thinking is that, by ripping open dead animals, the B. anthracis spores are given a chance to escape back into the soil and infect even more animals that might graze there later. Just one of many reasons scavengers get a bad rap.

Stopping the scavengers this way is very time-consuming, though -- just think of how big many parks can be, and how difficult it can be to track down every dead animal, and test each one -- so a team of ecologists from the University of Texas at Austin wondered if it was an entirely effective method for dealing with the problem. It turns out that the ripped-open-animals-spreading-anthrax hypothesis isn't quite correct.

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The study's lead author, Steven Bellan, said: "The hypothesis is that when a carcass is intact, the anthrax bacteria are forced into a kind of death match with putrefying bacteria from the gastrointestinal tract. But when the body is opened to the air, either by a scavenger or the haemorrhaging from all bodily orifices that occurs at death, the anthrax bacteria can escape that competition and more successfully produce spores."

To test the hypothesis, the team found seven zebras (and one wildebeest) dotted around Etosha National Park in northern Namibia that had died from anthrax infection.

Cages (with electric fencing) were erected around four of them, leaving them just as exposed to the elements but safe from being nibbled by any scavengers. The unprotected animals were left to be torn apart by scavenging beasts.

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Regular samples taken were from the soils around each animal to see if there was a bigger rise in the number of anthrax spores near to the scavenged animals. The results showed that "soil stained by terminally haemorrhaged blood and by non-haemorrhagic fluids exhibited high levels of B. anthracis spore contamination (ranging from 10^3-10^8 spores per gram) even in the absence of vertebrate scavengers" -- that means "scavenging by vertebrates is not a critical factor in the life cycle of B. anthracis and that anthrax control measures relying on deterrence or exclusion of vertebrate scavengers to prevent sporulation are unlikely to be effective".

Bellan said: "It appears that the anthrax bacteria can survive for some time in the carcass even though it may be competing with other bacteria. It also appears that fluids can escape from the carcass into the soil via mechanisms other than scavenging or through haemorrhages occurring at the time of death."

Bloating (caused by gases building up), and maggots, are two of the other things that can lead to skin leaking.

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However, it's only a small study in one part of the world, so its findings will have be further investigated before anthrax control policies are altered. Maybe, though, we should ease up on our general distaste for scavengers. They're not entirely disgusting.